1. Field of the Invention
The present invention relates generally to a vibration damping actuator for use as an active vibration damping apparatus and to an active vibration damping apparatus employing the same, and particularly to a vibration damping actuator suitable for use in a vibration damping apparatus such as an automobile engine mount, body mount, damper, or the like, and to an active vibration damping apparatus employing the same.
2. Description of the Related Art
In order to reduce vibration in an automobile body or other component that is very expected to be vibration-damped, there have been employed vibration damping devices that typically utilize vibration attenuating means, such as a shock absorber or rubber elastic body, or alternatively vibration isolating means that utilize the spring action of a coil spring, rubber elastic body, or the like. However, all of these vibration damping devices exhibit only passive damping action, resulting in the problem of an inability to exhibit adequate damping action in instances where, for example, the vibration to be damped has a characteristics such as frequency, that varies; or in cases where advanced vibration damping is required. Accordingly, in recent years, there have been developed and researched a number of active vibration damping devices that actively and in an offsetting manner reduce vibration to be damped, by means of exerting oscillating force on the component to be damped or the vibration damping device. Examples include those disclosed in Citations 1 and 2 listed hereinbelow.
In such active vibration damping devices, an actuator is needed in order to generate oscillating force, and the actuator needs to have highly controllable frequency and phase as regards the oscillating force generated thereby. A vibration damping actuator appropriate for use in an active vibration damping device may employ a coil, controlling electromagnetic force or magnetic force generated by means of controlling current flow to the coil. In order to actuate vibration of an output member in a high frequency range of several tens of Hz or greater, it is appropriate to employ a guide mechanism for guiding the output member in the actuation direction.
More specifically, as taught inter alia in Citations 1 and 2, a suitable design for such a vibration damping actuator typically has a guide hole extending on the center axis of a cup-shaped housing; an output member disposed spaced apart from the opening end of the housing, with the output member connected to the housing by an elastic connecting rubber part, and a guide rod disposed on the output member inserted into a guide hole; a coil member provided to either the housing or the output member; and an armature including a ferromagnetic body and/or permanent magnet disposed on the remaining housing or output member. By supplying electrical current through, the coil, oscillating force is exerted on the output member by the armature, causing the output member to undergo oscillating displacement in the center axial direction of the housing, on the basis of the guiding action of the guide rod by the guide hole.
In a vibration damping actuator of structure like that described above, for reasons having to do with the actuator assembly process or workability when making various adjustments of the actuator, the guide hole which is bored in the housing in order to guide the guide rod often has the structure of being bored through the housing and opening out onto the bottom face of the housing.
However, a problem is that when left with the guide hole open at the bottom face of the housing, foreign matter such as dirt, dust or water entering through the opening can infiltrate into the guided portion of the guide rod that is guided by the guide hole, hindering displacement of the armature and, consequently, of the output member, so that the output member does not consistently exhibit the desired actuating force.
To cope with this problem, it would be possible, as taught for example in Citation 3, to form a thread groove in the open area of the guide hole in the housing, and to screw a screw cap thereon to cover it. However, since it is the function of the actuator to generate vibration, where the guide hole is simply covered by a screw cap, there is an unavoidable risk of the screw cap loosening and coming off due to vibration. Additionally, with a simple screw cap tightening structure, it is not always an easy matter to ensure a consistently adequate level of sealing against water or the like.
Further, a screw cap naturally requires an operation to screw it in when attaching it, and since the screwing operation requires both considerable time and a tedious procedure, such a structure is needlessly laborious, and also creates the problem of an extremely laborious and time-consuming procedure to remove the screw cap during maintenance, for example. While it would be conceivable to install an O-ring in the threaded section of the screw cap in order to improve sealing, installation of an O-ring makes the structure even more complicated, and unavoidable results in an even more laborious screw cap assembly operation, and as such does not represent an effective solution.
As shown in FIG. 3 appearing in Citation 4, it would be possible to cover the guide hole by using several fastening bolts to affix a plate onto the opening of the guide hole in the housing. However, the process of threading several fastening bolts requires additional time and labor, and is not necessarily an effective method. Alternatively, it would be possible to weld the plate onto the opening of the guide hole in the housing, but since welding precludes subsequent removal, this creates the problem of being unable to subsequently perform adjustment or maintenance through the guide hole, and is not practical for this reason.
[Citation 1]                JP-A-9-89040        
[Citation 2]                JP-A-10-231886        
[Citation 3]                JP-A-2001-1765        
[Citation 4]                JP-A-9-49541 (FIG. 3)        